Wellbores are formed in subterranean formations for various purposes including, for example, extraction of oil and gas from subterranean formations and extraction of geothermal heat from subterranean formations. A wellbore may be formed in a subterranean formation using an earth-boring rotary earth-boring tool. The earth-boring tool is rotated under an applied axial force, termed “weight on bit” (WOB) in the art, and advanced into the subterranean formation. As the earth-boring tool rotates, the cutters or abrasive structures of the earth-boring tool cut, crush, shear, and/or abrade away the formation material to form the wellbore.
The earth-boring tool is coupled, either directly or indirectly, to an end of what is referred to in the art as a “drill string,” which includes a series of elongated tubular segments connected end-to-end that extend into the wellbore from the surface of the formation. Various tools and components, including the earth-boring tool, may be coupled together at the distal end of the drill string at the bottom of the wellbore being drilled. This assembly of tools and components is referred to in the art as a “bottom hole assembly” (BHA).
One common type of earth-boring tool used to drill well bores is known as a “fixed cutter” or “drag” bit. This type of earth-boring tool has a bit body formed from a high strength material, such as tungsten carbide or steel, or a composite/matrix bit body, having a plurality of cutters (also referred to as cutter elements, cutting elements, or inserts) attached at selected locations about the bit body. The cutters may include a substrate or support stud made of a hard material (e.g., tungsten carbide), and a mass of superhard cutting material (e.g., a polycrystalline table) secured to the substrate. Such cutting elements are commonly referred to as polycrystalline diamond compact (“PDC”) cutters.
Cutting elements are typically mounted on the body of a drag drill bit by brazing. The drill bit body is formed with recesses therein, commonly termed “pockets,” for receiving a substantial portion of each cutting element in a manner which presents the PDC layer at an appropriate back rake and side rake angle, facing in the direction of intended bit rotation, for cutting in accordance with the drill bit design. In such cases, a brazing compound is applied between the surface of the substrate of the cutting element and the surface of the recess on the bit body in which the cutting element is received. The cutting elements are installed in their respective recesses in the bit body, and heat is applied to each cutting clement to raise the temperature to a point high enough to braze the cutting elements to the bit body in a fixed position but not so high as to damage the PDC layer.
Unfortunately, securing a PDC cutting element to a drill bit restricts the useful life of such cutting element, as the cutting edge of the diamond table wears down as does the substrate, creating a so-called “wear flat” and necessitating increased weight on bit to maintain a given rate of penetration of the drill bit into the formation due to the increased surface area presented. In addition, unless the cutting element is heated to remove it from the bit and then rebrazed with an unworn portion of the cutting edge presented for engaging a formation, more than half of the cutting element is never used.
Rotatable cutting elements mounted for rotation about a longitudinal axis of the cutting element can be made to rotate by mounting them at an angle in the plane in which the cutting elements are rotating (side rake angle). This will allow them to wear more evenly than fixed cutting elements, having a more uniform distribution of heat across and heat dissipation from the surface of the PDC table and exhibit a significantly longer useful life without removal from the drill bit. That is, as a cutting element rotates in a bit body, different parts of the cutting edges or surfaces of the PDC table may be exposed at different times, such that more of the cutting element is used. Thus, rotatable cutting elements may have a longer life than fixed cutting elements. Additionally, rotatable cutting elements may mitigate the problem of “bit balling,” which is the buildup of debris adjacent to the edge of the cutting face of the PDC table. As the PDC table rotates, the debris built up at an edge of the PDC table in contact with a subterranean formation may be forced away as the PDC table rotates and new material is cut from the formation.